Subject: D8) What are "concentric eyewall cycles" (or
"eyewall replacement cycles") and why do they cause a hurricane's
maximum winds to weaken?

Contributed by Stan Goldenberg (HRD)

Hurricane WilmaOct. 19-21, 2005

"Concentric eyewall cycles" (or "eyewall replacement cycle"
) naturally occur in intense tropical cyclones
, i.e. major hurricanes (winds > 50 m/s, 100 kt, 115 mph) or
Categories 3, 4, and 5 on the Saffir-Simpson scale.
As tropical cyclones reach this threshold of intensity, they usually -
but not always - have an eyewall and radius of maximum winds
that contract to a very small size, around 10 to 25 km [5 to 15 mi].
At this point, some of the outer rainbands may organize into
an outer ring of thunderstorms that slowly moves inward and robs
the inner eyewall of its needed moisture and momentum. During
this phase, the tropical cyclone is weakening (i.e. the maximum
winds die off a bit and the central pressure goes up). Eventually
the outer eyewall replaces the inner one completely and the storm
can be the same intensity as it was previously or, in some cases,
even stronger. A concentric eyewall cycle occurred in Hurricane
Andrew (1992) before landfall near Miami: a strong intensity was
reached, an outer eyewall formed, this contracted in concert with
a pronounced weakening of the storm, and as the outer eyewall
completely replaced the original one the hurricane reintensified.
Another example is Hurricane Allen (1980) which went through
repeated eyewall replacement cycles -- going from Category 5 to
Category 3 status several times. To learn more about concentric
eyewall cycles, read Willoughby et al.
(1982) and Willoughby (1990a).

It was the discovery of concentric eyewall cycles that was
partially responsible for the end of the U.S. Governments's
hurricane modification experiment
Project STORMFURY, since what the scientists had hoped to
produce through seeding was happening frequently as a natural part
of hurricane dynamics.